1. Field of the Invention
The present invention relates to a solid polymer electrolyte fuel cell system and more particularly to an improved fuel cell structure and method for feeding water of inclusion in a solid polymer electrolyte membrane and for feeding reaction gases for the fuel cell system.
2. Description of Prior Art
Fuel cells are roughly classified into two groups, for example, low temperature-operating ones such as an alkali type, a solid polymer electrolyte type, and a phosphoric acid type, and high temperature-operating ones such as a molten salt type, and a solid oxide electrolyte type.
Solid polymer electrolyte type fuel cells have a solid polymer electrolyte membrane having two main surfaces provided with an anode and a cathode, respectively, and an electrode substrate, the anode and cathode being sandwiched by the solid polymer electrolyte and the respective electrode substrates. As the solid polymer electrolyte membrane, there have been used polystyrene cation exchange resins having a sulfonic acid group as cationic electroconductive membranes, fluorocarbon sulfonic acid/polyvinylidene fluoride mixed membranes, or membranes composed of a fluorocarbon matrix and trifluoroethylene grafted thereto.
Recently, fuel cells with a prolonged service life by the use of perfluorocarbonsulfonic acid membrane (Nafion, trade name for a product by DuPont de Nemours, Ill. U.S.A.) have been put on the market. Solid polymer electrolyte membranes have proton (hydrogen ion) exchange groups in their molecule and have resistivities not higher than 20 .OMEGA..multidot.cm at room temperature when hydrated to saturation, thus acting as a proton-conductive electrolyte. Saturated water content varies reversibly depending on the temperature of the membrane.
An electrode substrate, which is made of a porous material, acts as a means for supplying a reaction gas to a fuel cell and also as a collector. In anodes or cathodes, there are formed three phase zones where electrochemical reactions takes place.
In an anode, there occurs the following reaction: EQU H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- ( 1)
In a cathode, what occurs is the following reaction: EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O (2)
In other words, in the anode, hydrogen supplied from outside of the system produces protons (H.sup.+) and electrons (e.sup.-). Protons produced migrate through the ion exchange membrane toward the cathode whereas electrons migrate into the cathode through an external circuit connected thereto. On the other hand, in the cathode, oxygen supplied from outside of the system, protons which have migrated through the ion exchange membrane and electrons transferred through the external circuit react to produce water (H.sub.2 O).
In this type of solid polymer electrolyte fuel cells, protons migrate from the anode to the cathode through the ion exchange membrane in a hydrated state, resulting in that the water content of the membrane in the vicinity of the anode decreases and the ion exchange membrane tends to be dried. Hence migration of protons becomes difficult in the vicinity of the anode unless water is supplied thereto. On the other hand, in the cathode, water is produced as shown by formula (2) above. However, generally solid polymer electrolyte fuel cells are operated at temperatures not higher than 100.degree. C., which means that water produced on the cathode side is considered to be in a liquid state. Therefore, in the cathode, excess amounts of water accumulate since water is not only freshly produced as a result of electrode reaction but also released from hydrated protons due to disappearance of the protons in the reaction on the cathode. The water which has accumulated would fill and clog pores in the electrode substrate to inhibit diffusion of the reaction gas therethrough.
Accordingly, in order to operate a solid polymer electrolyte fuel cell continuously and efficiently, it is necessary to properly supply water to the anode to replenish water of inclusion contained by the solid polymer electrolyte membrane and discharge the water which has migrated therefrom and accumulated in the cathode. For optimizing the water content of the ion exchange membrane, the water of inclusion in the ion exchange membrane has conventionally been replenished by bubbling the fuel gas into water kept at a temperature higher than the temperature at which the fuel cell is operated to humidify the fuel gas and supplying the gas thus humidified to the anode side of the fuel cell. On the other hand, the water which accumulated in the cathode has conventionally been discharged by supplying a large amount of dry oxidizer gas to the cathode of the fuel cell, or by cooling steam formed in the cathode to condense it and discharging the resulting water to outside the system.
However, the conventional method in which water in the form of steam is supplied to the ion exchange membrane to replenish therewith the water of inclusion in the membrane has some problems. For example, the conventional method does not supply an amount of water which is enough to replenish the water which has migrated due to hydration because water condenses in the inside of the ion exchange membrane in an amount which corresponds to the difference between the saturation vapor pressure of the reaction gas at a humidification temperature and the saturation vapor pressure at a cell operation temperature. Thus, it is generally difficult to use a large difference between the humidification temperature and the cell operation temperature. Further, use of increased humidification temperatures leads to an increase in the partial pressure of aqueous vapor (0.47 atm at 80.degree. C.; 0.69 atm at 90.degree. C.) to thus decrease the partial pressure of the fuel gas. As a result, supply of the fuel gas decreases to deteriorate the characteristics of the fuel cell.